NSWC/WOL/TR 75-35 



Cyanide can be present as free cyanide ion and as dissolved 

 HCN gas. The cyanide electrode responds only to the free cyanide 

 ion, so the sample must be adjusted to the corrected pH range before 

 measurement . 



Sample pH adjustment before measurement is effected by adding 

 concentrated NaOH to the sample to make it . 1 M in hydroxide ion. 

 This will make the sample pH about 13, insuring that substantially 

 all the HCN is converted to free cyanide ion. A calibration curve 

 for the cyanide electrode is made using a series of standardizing 

 solutions of different concentrations. Each standard solution is 

 made 0.1 M in hydroxide ion so that the NaOH treated samples and 

 standards will have about the same ionic strength and pH. 



The electrode does not respond to most common ions in water, 

 such as fluoride, chloride, nitrate, sulfate, phosphate, carbonate, 

 sodium, potassium, and ammonium. Sulfide and iodide ions are the 

 only major interferences. 



4.3.1.3 Colorimetric Method 



Chlorine in water may be present as free available chlorine 

 (in the form of hypochlorous acid and/or hypochlorite ion) or as 

 combined available chlorine (chloramines and organic chloro deriva- 

 tives) . Chlorine in aqueous solution is not stable, and the chlorine 

 content of samples, particularly weak solutions, will rapidly 

 decrease. Exposure to sunlight or other strong light or agitation 

 will accelerate the reduction of chlorine present in such solutions. 

 Therefore, it is recommended that chlorine determinations be started 

 immediately after sampling, avoiding excessive light and agitation. 



Several methods have been described for the determination of 

 free or combined available chlorine in water. 21 The colorimetric 

 method using orthotolidine appears suitable for field use. In this 

 method, the water sample is mixed with a 2N HCl solution containing 

 0.15% orthotolidine (the ratio by weight of orthotolidine to chlorine 

 must be at least 3:1). A yellow holoquinone color is developed, and 

 the color intensity is immediately measured with a portable colori- 

 meter (Hach Chemical Co., Ames, Iowa; Model DR-EL, $330). The 

 minimum detectable concentration of chlorine is approximately 10 Vg/l. 



"? 1 



Standard Method for Examination of Water and Wastewater, 1971, 13th 



Edition, American Public Health Association, Washington, D. C. 



47 



